All teas are really healthy for you and all of them have similar antioxidant levels. White tea has the highest (only by a tiny margin) due to the fact that it is unfermented. Green tea would be next, then black. All are very close though, so drink anything that you like.

If you are used to coffee, you might want to start with a black tea, since it will be closest to what you are used to. Assam or Yunnan Gold should both produce a good, strong flavor.

The order of flavor strength in tea goes something like black -> oolong -> green -> white. However, there is a lot of variation in there, so some greens might be stronger than some oolongs.

Or you could just try several different types of tea, and look for a flavor you like that is not neccessarily anything like coffee. Basically, rather than finding a substitute for coffee, you would be finding another flavor you like. Green teas tend to be more vegetal, while black teas tend to be more malty. White teas are the most subtle. If you like jasmine, jasmine teas (listed on Adagio's site under oolong) can be quite tasty.

It also seems like a lot of former coffee drinkers prefer flavored teas to the plain ones, at least at first.